THIS INVENTION relates, broadly, to a separator for a high temperature rechargeable electrochemical cell. More particularly, the invention relates to a method of making such separator.
According to the invention there is provided a method of making a ceramic solid electrolyte separator for a high temperature electrochemical cell from a particulate starting material capable of being sintered to form a unitary polycrystalline solid electrolyte ceramic artifact, the method comprising the method steps of:
admixing the particulate starting material with a binder to form an extrudable mixture;
extruding the extrudable mixture to form a unitary sinterable green artifact including a multiplicity of at least five tubes interconnected together in side-by-side relationship; and
sintering the green artifact to form a sintered polycrystalline ceramic solid electrolyte artifact including a multiplicity of at least five ceramic solid electrolyte separator tubes interconnected together by sintering and arranged in side-by-side relationship.
Conveniently the solid electrolyte is a conductor of sodium ions, such as xcex2-alumina or, preferably, xcex2xe2x80x3-alumina. The particulate starting material may be in the form of particles of the solid electrolyte material, or it may be in the form of a precursor mixture which, upon heating thereof to sintering temperatures, is converted or transformed to the solid electrolyte material. When the solid electrolyte is to be xcex2- or xcex2xe2x80x3-alumina, it may be a powder mixture including a suitable oxide or hydroxide of aluminium, together with soda and a constituent selected from lithia or magnesia, of a type known in the art to form xcex2- or xcex2xe2x80x3-alumina when heated to sintering temperatures. Precursors may be used of the alumina oxide, of the soda, of the lithia or of the magnesia, being substances which, when heated in air to a temperature of 700xc2x0 C., become the oxide in question. In this regard aluminium hydroxide is in fact a precursor of aluminium oxide of this type. In a particular embodiment of the invention, thus, the particulate material may be a precursor of a solid electrolyte selected from the group of solid electrolytes consisting of sodium xcex2-alumina, sodium xcex2xe2x80x3-alumina and mixtures thereof, the binder being selected from the group consisting of binders having thermoplastic properties, binders having thermosetting properties, binders having both thermoplastic properties and thermosetting properties, and mixtures of such binders.
The binder may be an organic binder having both thermoplastic properties and thermosetting properties, having thermoplastic properties when initially heated, but when heated to higher temperatures than those at which it is thermoplastic, becoming thermosetting in nature. Suitable binders, in the form of binder formulations, are described, for example, in British Patent GB 1 274 211. As mentioned in GB 1 274 211, a single binder compound may be used, provided that it displays the required thermoplastic and thermosetting properties.
A suitable binder is polyvinyl-butyral, as it has both thermoplastic properties and thermosetting properties, and it may be used together with a plasticizer such as dibutyl phthalate and a solvent such as methylethyl ketone, the solvent and plasticizer facilitating blending of the binder into the particulate starting material such as xcex2- or xcex2xe2x80x3-alumina or a precursor mixture thereof, to form a sufficiently homogeneous extrudable mixture. However, if a high energy mixer, such as a Banbury mixer, is used, the plasticizer and solvent can, in principle, be omitted.
In practice the extrusion may be to provide the multiplicity of tubes of the green artifact in what amounts to be a closely-packed side-by-side abutting relationship, in which the peripheries of the tubes abut and are integral with one another, or, instead, the tubes may be spaced from one another in side-by-side relationship by longitudinally extending spacers in the form of webs of extruded material, each web having side edges integral with the peripheries of two adjacent tubes. In this regard, by a multiplicity of tubes is meant, at least 5 tubes, typically at least 10 tubes and preferably at least 25 tubes. Thus, in one embodiment, the extruding may act to provide the tubes of the green artifact in a close-packed side-by-side abutting relationship in which the peripheries of adjacent tubes abut one another and, after the sintering, are integral with one another; and, in another embodiment, the extruding may act to provide the tubes of the green artifact in a mutually spaced side-by-side relationship, the tubes being spaced apart from one another by longitudinally extending spacers in the form of webs of extruded material, each web having side edges integral with the peripheries of two adjacent tubes.
It is contemplated that extrusion will preferably be intermittent, a plurality of green artifacts being successively extruded from a batch of extrudable mixture and detached from the remainder of the batch, for example by being cut or sliced from material of the batch, in a direction transverse to the tubes. Each green artifact will be intended to form, after sintering, a cell separator. Typically a plurality of green artifacts will be extruded from a batch of extrudable mixture. In a particular embodiment, the extruding may be intermittent, being of a plurality of unitary sinterable green artifacts in succession from a batch of the extrudable mixture, a plurality of the green artifacts being sintered simultaneously in a single sintering step.
In use the separator may form part of an assembly comprising a header (for connection of the separator to a cell housing), at least part of which header may be electronically and electrochemically insulating, the header being hermetically connected to the open ends of the tubes, with its interior in communication with the interiors of the tubes for example as described in International Patent Application PCT/GB98/00389 published as WO 98/35400, by glassing, ie glass welding. The sintered separator may thus be glass welded to a suitable header having electronically- and electrochemically insulating properties. The end of the separator opposite or remote from the header will have a plurality of openings leading into the tubes or into spaces between the tubes and extending alongside the tubes, and certain of the tubes will typically be closed off by one or more sintered closures of the same material as the tubes. While, in principle, these closures may be plugs or panels of the solid electrolyte material in question, glass welded to the tubes, it is expected that it will be preferred to form, eg by moulding, the closures simultaneously with the extrusion of each artifact, from the extruded material. Thus, in general, the method may include, after the sintering, hermetically connecting a header to open ends of a plurality of the separator tubes at one end of the separator, the header having an interior in communication with the interiors of these tubes, the hermetic connecting being by glass welding and the header being both of a material which is electronically insulating and electrochemically insulating, and the method further including closing off the ends of a plurality of the separator tubes at the end of the separator remote from the header. In this case, the closing off of the ends of the plurality of tubes at the remote end of the separator may be by moulding the ends of said tubes while they are in a green state, to form closures which close of said ends of the tubes, the moulding taking place, prior to the sintering, while the separator is being extruded.
According to this feature, the downstream ends of the tubes, ie those which emerge or are formed first during the extrusion, may be moulded shut while in plastic and extrudable/mouldable state, during the initial part of the extrusion step. It is contemplated, in this regard, that an extruder having a die with a multiplicity of more or less annular openings or orifices for extruding the tubes may be used, together with a movable mould for moulding said ends of the tubes shut. The mould may have a working face provided with a multiplicity of recesses or indentations, one for each tube, the recesses or indentations being arranged in an arrangement corresponding to the arrangement of the annular orifices in the die, and corresponding to the arrangement of the tubes in the separator.
In use the mould will be placed in a starting position at or closely adjacent the extruder at the start of the extrusion of each separator, the mould being arranged so that the recesses or indentations of the mould register with, and are opposed to, the outlets of the annular orifices of the die, so that the tubes will be extruded respectively into the recesses or indentations, at the start of the extrusion. Extrusion is continued until the material of the mixture entering the recesses or indentations is deformed to close off the downstream ends of the extruded tubes, after which the mould is moved away from the die in the direction of extrusion to permit the remainder of the green artifact comprising the tubes and separator to be extruded, after which the separator is cut or sliced, at or adjacent the die orifice outlets, to detach the separator from the mixture in the extruder. The extrusion in the form of a green artifact is removed, the mould is returned to its starting position and the extrusion/moulding cycle is repeated for the succeeding separator. The extrusions can then be sintered before being glassed to their headers.
In other words, the moulding may be by means of a mould having a multiplicity of recesses, there being a recess for each tube of the green artifact, the mould being aligned with the tubes during the extrusion so that each tube registers with a corresponding one of the recesses, the extrusion of the tubes being into the recesses so that the leading ends of the tubes are received in the recesses and are closed off by being moulded shut by contact with inner surfaces of the recesses; and the tubes may be extruded from a batch of the extrudable mixture into the recesses at the start of each extrusion step, with the mould stationary and located in a starting position until the tubes are moulded shut, the mould then being moved away from the starting position in the direction in which the tubes are extruded, until the tubes are fully extruded, the green artifact then being detached from the unextruded remainder of the batch, and the mould then being returned to the starting position.
The tube arrangements for the separator include rectangular close-packed or rectangular spaced arrangements, and hexagonal close-packed or hexagonal spaced arrangements. The tubes in turn may have various cross-sections, as desired. Thus the tubes may be of circular cross-section, spaces being defined between the tubes, whether the tubes are spaced or close-packed, or the tubes may be square, rectangular or hexagonal in cross-section, in which case they will usually be close-packed with no spaces therebetween, although they may in principle be spaced by spacer webs from one another.